How would explain the enormous numbers of serious bugs, or the fact the Safari had slipped well behind Firefox, a non-profit with a fraction of the resources?
I lost count of the number of times local storage or indexedDB was broken by updates. Not to mention countless, very obvious regressions. Web Apps that would randomly crash 60 times a day (with one time requiring us to wait 1-2 months for the patch to role out.
Also even in the code base, you'd have (exceptionally talented) engineers juggling complete disparate and unrelated areas of code.
One major issue with no competition, is Apple never responded to developer needs. There were literally many thousands of requests for Push API for years and since no other vendor could implement them (no other vendor could even install Web Apps), there was no competitive pressure.
I don't buy the "user experience" argument. A browser full of bugs does not lead to a good user experience and the expense it imposes on developers is enormous.
As a personal example, we had such serious issues with scroll, tap to focus and bounce that the only solution was to recode all scroll-logic including scroll-bars, momentum bounce in javascript while the same app could be built with only minor issues in both Firefox and Chrome despite the fact that iOS Safari was our primary target.
As for the bugs, they just didn't get fixed. So many for, years and years.
That all said, this is past history, and I'm very hopeful for the future. The fact Apple has hired a ton more staff (including significant numbers of QA/Testers staff), a ton of really excellent web OG's and are pumping through both the features and the bugs really bodes well for the future of browser competition.